If you have other effects applied, remove or nest them into a sequence/composition. Denoiser needs to work directly with your frame information, and having other effects applied can affect its ability to de-noise a frame or pull a sample. Either nest your effects with the clip into their own composition, or remove them altogether.

Make sure the clip matches the sequence/composition settings exactly. If your footage is 23.97fps, and the composition is set to 24 fps, that can affect the plugin's ability to pull an accurate frame or denoise the footage.